The invention relates to a shared use of different wireless telecommunication systems and terminals thereof, and particularly to a method of extending coverage areas of private networks.
A method of the invention is applicable to any wireless telecommunication network and terminals thereof. The invention is particularly well suited for use in private networks of the Private Mobile Radio or Professional Mobile Radio PMR type, i.e. trunking networks, which are typically company networks or official networks wherein all channels are used by one or more companies or official organizations. As digital networks attract the most interest in the development of wireless telecommunication systems, the company networks have also been provided with digital standards. An example of a digital radio telephone system or mobile communication system whereto the present invention can be applied is a Trans-European Trunked Radio or Terrestrial Trunked Radio TETRA system. The implementation of the TETRA system is described in the standard ETS 300 392-2, March 1996, Radio Equipment and Systems (RES); Trans-European Trunked Radio (TETRA); Voice plus Data (V+D) Part 2: Air Interface (Al), ETSI, 583 pages.
The TETRA is a mobile communication system which operates at a 380 to 520 MHz radio frequency band and uses Time Division Multiple Access TDMA modulation. A 56.67 ms long TDMA frame is divided into four time slots, i.e. one frame can comprise four traffic channels. Depending on channel coding, the data transmission rate of a single traffic channel may vary between 2.4 and 7.2 kbit/s, and by allocating all channels of one frame to a single data transmission connection, a maximum rate of 28.8 kbit/s is achieved. Assignment of speech items between terminals and base stations is controlled by a Switching and Management Infrastructure SwMI comprising digital exchanges DXT to which the terminals are connected. Companies and official organizations may reserve channels from the TETRA infrastructure for their use, in which case the area-based reservation of the channels is typically restricted to the operating range of the organization.
A group call is one of the main functions of a PMR network. In addition to their own subscriber numbers, in the PMR network subscribers are provided with group numbers to indicate the group call group or subscriber group the subscriber belongs to, which enables calls intended for all subscribers in the group to be transmitted to the subscribers in the group. The group call is used e.g. when several participants are involved in an event, and particularly when the entire group continually needs to know the current situation. The group call is a conference call wherein all participants may take turns at speaking, i.e. be provided with speech items, and listening to each other. The system makes sure that only one participant may speak at a time. The subscriber may belong to several group call groups or subscriber groups whose group calls the subscriber can monitor simultaneously and switch a group call with a desired priority, or a group call with the highest priority as far as the subscriber is concerned, to be received. In the group calls, the entire group is called by using a single call number. As to the radio path, the group call can be implemented in a simplex form as a point-to-multipoint call wherein speech at a given time travels from one speaker to several receivers and the speech item is assigned to a next speaker according to a predetermined practice. Another typical way to implement a group call is a semi-duplex call wherein the speakers request speech items e.g. by pressing a particular switch. A call may comprise one or more speech items, or the speech items may be sequential. Naturally, it is also feasible to implement the group call as a duplex call wherein all participants are allowed to speak simultaneously.
In public radio telephone networks, such as a Global System for Mobile Communication GSM system, all calls are typically provided with the same priority, i.e. no user is given priority over any other user. In the private radio networks, i.e. the PMR networks, the operation of the network can be controlled such that users carrying out special e.g. emergency-related maintenance tasks or other such important tasks are provided with higher priority, i.e. their high-priority call will be switched first in a high-volume traffic situation. An example of a high-priority call is an emergency call, which refers to a call wherein at least one subscriber is in distress. The subscriber in distress can activate the emergency call, or it can be activated by another subscriber station or dispatcher monitoring the operation. When necessary, the emergency call can interrupt other calls in order to obtain the speech channel it needs.
A problem with the arrangement described above is the limited coverage of the mobile communication networks, such as the PMR networks, or the geographical area reserved therefrom for the use of an organization, and, on the other hand, the incompatibility of the terminals of a particular mobile communication system, such as PMR terminals, with other telecommunication systems. On many occasions, however, a terminal should establish a connection to its own network from outside the coverage area of the network, e.g. a PMR terminal should participate in a group call while being located outside the coverage area of the PMR network. In such a case, the PMR terminal is typically located in the coverage area of another wireless telecommunication network, such as the GSM network, but the incompatibility of the PMR terminal with the GSM will not allow a connection to be established to the PMR system. In addition, managing the group call essential to the PMR system in public mobile communication networks differs considerably from managing the group call in the PMR networks, which means that participating in a PMR group call by using a connection established through another system would require drastic changes in the standardization of the public mobile communication networks. A further drawback of the PMR networks is the rather low data transmission rate e.g. when video image from a site of an accident is to be transmitted in an official network. The video image could be transmitted through another, more broadband network but this is impossible due to the incompatibility problems.
Publication WO 99/48312 describes an arrangement wherein a terminal in a wireless telecommunication network is able to establish a data transmission connection to said telecommunication network both wirelessly through a radio interface specified for said telecommunication network and through a wired connection using an Internet Protocol IP. Existing wired public telecommunication connections, such as the Internet, and private connections, such as company local area networks, can thus be utilized for efficient data transmission between wireless terminals, thus extending the coverage area of the wireless telecommunication network everywhere an Internet connection can be established. The connection of the terminal to an IP network and therethrough to the wireless telecommunication network is established through a particular base station emulator PBU, which arranges messages according to the wireless telecommunication network protocol into IP data packets, which are decompressed at the other end of the IP connection into messages according to the wireless telecommunication network protocol before being transmitted to said wireless telecommunication network. The problem with the arrangement is, however, that the implementation is tied up with the wireless telecommunication system and the transmission of the messages according to the protocol used therein on top of the IP protocol. Thus, the arrangement does not enable a shared use of terminals which belong to different telecommunication systems. A further problem is the required connection establishment to the base station emulator, which considerably restricts the number of potential connection setup points.